Problem: On the first 4 quizzes of her history class, Stephanie got an average score of 85. What does she need on the next quiz to have an overall average of 88?
Let her score on the next quiz be $x$ The sum of all of her scores is then $4 \cdot 85 + x$ The same sum must also be equal to $5 \cdot 88$ Solve: $x = 5 \cdot 88 - 4 \cdot 85 = 100$.